Support surfaces have a critical role in modern society. This is especially true of support surfaces for wheelchairs and beds. The support surface in a wheel chair is commonly called a seat or “seat cushion.” The support surface on a bed is commonly called a bed or mattress.
For those users who must spend large amounts of time in a wheelchair or bed, the support surface should achieve a number of objectives. First, the support surface should maximize user function. This includes maximizing the user's ability to maneuver and to engage in activities. Second, the support surface should be comfortable for the user. Third, the support surface should be reliable and durable. Fourth, the support surface should be easy to clean and maintain. Moreover, the support surface should be safe for the user. Many aspects of support surface design can simultaneously affect user comfort, function, and safety. For example, if a user is not stable upon the support surface, the user likely will not be comfortable, will not have adequate function, and will not be safe.
When a user has paralysis, decreased sensation or absent sensation, a particular danger can be the formation of decubitus ulcers (commonly known as “pressure ulcers” or “bed sores”). Decubitus ulcers are lesions that form on parts of the body that are in ongoing contact with objects such as a mattress, seat cushion or other support surface. The symptoms of decubitus ulcers range from skin redness (stage I) to “tunneling ulcers” with necrosis of the skin, fat, muscle and even bone (stage IV).
Decubitus ulcers are of major concern to the afflicted patients, their caregivers, and the medical community. The scale of the problem is immense. It is estimated that approximately 1.2 million people are suffering from decubitus ulcers at any one time in the United States alone. It is reported that there are 60,000 deaths annually from complications arising from decubitus ulcers and the current cost to the U.S. Health Care System to treat these and other associated conditions is estimated at US$15-$40 billion annually.
Wheelchair and bed users can face a truly daunting challenge in trying to prevent and manage decubitus ulcers. Decubitus ulcers can lead to hospitalization, plastic surgery, and even amputation. Once a patient has had an ulcer with skin scarring, the risk of future ulcers increases. Those afflicted can face a repeating cycle of ulcer formation, hospitalization and surgery.
The repeated insult to the body, however, is only part of the affliction. Hospitalization and long-term rest can destroy families and social networks and severely hamper work and leisure. Costs incurred because of decubitus ulcers can be dramatic as well. In some cases, a single patient can incur ulcer-related medical costs that go well beyond one million dollars. Indirect costs such as lost productivity increase this monetary burden.
The general reason wheelchair bound and bed ridden patients face problems with decubitus ulcers is clear. Prolonged lying upon a mattress or sitting upon a seat cushion, especially with no ability or limited ability to move, creates prolonged pressure and shear/friction loads on the body, thereby leading to ulceration. A mobile person turns frequently while sleeping or shifts while sitting; this relieves shear & pressure loads and provides for healthy blood circulation. In an immobile patient, pressure loads can much more easily occlude blood flow and lead to tissue damage. As shown in FIG. 5, some regions of the body that are often affected in bed ridden patients are tissues near bony areas such as cranium 390, scapula 391, sacrum 392, ischial tuberosities (149a, 149b in FIGS. 2 and 3), elbow and heel bone (calcaneus) 394. As shown in FIG. 2, wheelchair users are generally affected in tissue near bony areas such as the sacral region 143; coccyx; ischial tuberosities 149a, 149b; and greater trochanters 147b. These regions are commonly referred to as “bony prominences.”
The traditional way to avoid formation of decubitus ulcers in bed users is for a family member, caregiver or institutional employee to regularly turn (it is recommended that this be done every two hours) and stabilize a patient in a new position to relieve pressure loads and re-establish blood flow. This has to be done around the clock and has a considerable number of drawbacks.
Some prior art wheelchair cushions and bed mattresses attempt to prevent ulceration by equalizing pressure loads over a body. Wheelchair cushions made from thick sections of foam attempt to equalize pressure loads using a very compressible surface. Water beds attempt to equalize pressure loads using a fluid medium. However, even these devices still lead to ulceration in critical areas.
Some prior art beds attempt to mimic the natural turning of a mobile person by automatically tilting and/or rotating the support surface. Other prior art beds inflate and deflate internal air bladders to vary the location of pressure loads over time. These beds attempt to prevent ulceration by constantly changing which tissues are subject to the greatest pressure loads.
Another method for avoiding formation of ulcers is to carefully fit a support surface to the user. This careful fitting distributes pressure loads in a way that minimizes pressure in critical areas and raises pressure loads in more tolerant areas. Ideally, no areas are subject to pressure loads that would lead to occlusion of blood flow. However, an expensive custom support surface is usually required. Prior art support surfaces have been made, for example, by custom shaping large blocks of foam into mattresses or seat cushions.
The prior art practice of shaping large blocks of foam is an iterative, expensive, and time consuming process that requires a skilled fitter. The typical prior art process uses a plaster cast of a patient's body to mold the foam. Modifications are made to the plaster cast so that pressure relief will be incorporated into certain areas of a custom fit support surface. However, these modifications cannot be checked until after molding.
Changes in patient's body shape or mass, such as is common in disabled populations due to atrophy or weight loss, can alter the required support surface shape. When a new support surface shape is required, the above process must be repeated. Moreover, even newly made prior art custom fit support surfaces sometimes fit poorly and require additional modification.
Fitting a custom support surface is an extremely difficult process that has many variables and is very individualized. For example, a fitter must consider the user's gender, size, weight, disability, deformities, personal preferences, and subjective comfort. To minimize the risk of decubitus ulcers, it is imperative that the user's support surface fits properly. However, even professional fitters often lack the finances, options, time and knowledge to provide an ideally fit support surface. Moreover, because health insurance reimbursement is poor for custom fitting mattresses, seat cushions and other support surfaces, there is a resulting reluctance by professionals to perform this type of work. As a result, poorly fit support surfaces often lead to an increased risk of ulceration.
One prior art technique requires making an impression, making a plaster cast from the impression, and modifying the cast after curing by adding or removing plaster. The fitter uses the modifications to customize the seating pressure or fit of the support surface in various ways. A support surface is typically molded from the modified plaster cast, thus the final support surface reflects the modifications to the plaster cast. However, modifications to plaster casts are not always ideal. After test fitting a custom-fit support surface, sometimes additional modifications are needed to meet the needs of the user for a good fit, comfort and safety; in that case, another custom molded support surface must be made and again test-fitted. This process of trial and error is time consuming and expensive.
Test fitting often involves the use of a pressure mat to determine if a support surface is performing adequately. Pressure mats commonly used in the wheelchair seating industries often read pressure values in millimeters of mercury. Determining acceptable pressure values for a sitting support surface is a subjective endeavor that depends upon the needs of each patient. As a general guideline, it is common that a pressure of 100 millimeters of mercury, especially in a critical area, could be considered excessive while a pressure of 40 to 60 millimeters of mercury is more likely to be an acceptable value.
Another prior art method of creating a custom fit support surface uses CAD (computer aided drafting)/CAM (computer aided manufacturing) techniques. Shape data is collected by scanning a previously taken plaster cast impression or by directly scanning a person's body. This shape data is modified electronically using a CAD system such that the final custom support surface will incorporate pressure relief in critical areas. A custom fit support surface can be manufactured semi-automatically by a robotic machining center. Typically this robotic machining center is a computer numeric control mill. This prior art method suffers many of the problems of previous methods. Modifications to the custom fit support surface are still being made on a trial and error basis. Only after the custom fit support surface is manufactured can it be test fit to the patient. If the electronic modifications prove to be less than ideal and further modifications to the custom fit support surface are needed, another custom fit support surface must be manufactured and the original discarded.
Most prior art support surfaces utilize a mattress or other type of cushion to support the user. However, mattresses and cushions are typically good insulators, lack breathability, and do not distribute pressure ideally. Moreover, bed sheets, seat cushion covers and clothing materials commonly used with such supports are usually not designed or selected to minimize frictional forces. When “local factors” such as pressure, shear, heat, and moisture rise, the rate of tissue damage leading to ulcer formation increases. Ideally pressure, shear stresses, excess temperature increases and moisture should be minimized.